1. Field of the Invention
The present invention relates to electron beam devices, and more particularly, to an electron tube amplifier utilizing trajectory modulation of an electron beam.
2. Description of Related Art
Electron tube amplifiers are well known in the art for converting the energy of an electron beam to microwave energy in response to an RF drive signal. In a typical linear beam electron device, such as a traveling wave tube (TWT) or klystron, an electron beam originating from an electron gun is caused to propagate through an RF interaction structure. At the end of its travel, the electron beam is deposited in a collector that captures the remaining energy of the spent electron beam. The beam is generally focused by magnetic or electrostatic fields in order for it to be effectively transported from the electron gun to the collector without loss to the interaction structure. In a TWT, for example, an RF wave propagates through a helical structure or set of cavities that comprise the interaction structure, coupling to the electron beam such that the beam gives up energy to the propagating wave. In contrast, the interaction in a klystron is discrete rather than continuous. Thus, the electron device may be used as an amplifier for increasing the power of a microwave signal.
Linear beam electron devices use either velocity or density modulation to establish an AC current in the electron beam that is subsequently converted to RF energy at the output of the device. Velocity modulation works by alternately accelerating and decelerating a beam of electrons passing through an RF driven input structure, such as a cavity or traveling wave circuit. As the electrons drift downstream, their velocity differences cause them to group at the RF frequency. RF current is then induced at the output of the device as the resultant electron bunches pass through. High gain can be achieved by adding circuit components to reinforce the velocity modulation imparted at the input section. When driven to saturation, device efficiency, i.e., the degree to which the DC electron beam power is converted to RF energy, can approach 70 percent. However, in the linear region of operation, a significant portion of the DC beam is not converted to RF, which tends to compromise efficiency. Also, the circuit length required to translate velocity to current modulation is often substantial. This is particularly true in low frequency applications, where velocity modulated amplifiers can be several meters long.
Density modulation works by RF gating the electron flow directly from the cathode surface, accelerating the resulting electron bunches, and extracting power using an output section. As a consequence, density modulated devices are generally considerably shorter than their velocity modulated counterparts. Additionally, because electron emission is controlled by the RF drive level, density modulated devices retain a high degree of efficiency even when operated in the linear region. This characteristic is the reason that inductive output tubes using density modulation have replaced klystrons for UHF television broadcast.
In nearly all density modulated devices, RF gating of the electron emission from the cathode is accomplished via an input cavity structure with a high electric field region situated between the cathode surface and a control grid. The gain of these devices is limited due to the fact that a substantial amount of input power is required to develop an electric field sufficient to draw a moderate amount of electron beam current. The control grid is spaced very close to the cathode, not only to enhance the electric field at the cathode surface, but also to limit the transit angle of the electrons. The transit angle consideration constrains the operation of these devices to the lower frequency end of the microwave spectrum. Devices that use grids for RF modulating the electron beam are also limited in power due to control grid interception.
Therefore, to fully exploit the benefits provided by density modulation, i.e., high efficiency and compact size, without the consequent frequency, power, and gain limitations, it would be desirable to provide a method and apparatus for gating electron current at microwave frequencies that does not rely on a closely spaced control grid.